This invention relates to steelmaking and more particularly to a method and apparatus for controlling the rate and amount of injection of flux into a steelmaking vessel as needed. Still more particularly, the invention relates to a method and apparatus for controlling the injection of flux into a steelmaking furnace, such as a Q-BOP furnace or the like. The basic Q-BOP furnace and process itself are disclosed in U.S. Pat. No. 3,706,549 issued Dec. 19, 1972 to H. Knuppel et al for "Method for Refining Pig-Iron Into Steel."
In a conventional Basic Oxygen Process for refining steel, oxygen is blown into a vessel through a lance positioned above the iron melt. While this process is satisfactory for many purposes, the mixing of the bath is not complete enough for some applications, iron losses are relatively high and only a portion of the oxygen issuing from the lance is utilized. An improved process for refining steel employs oxygen blown from below the surface of the melt resulting in better mixing, higher efficiency and less smoke generation than the conventional method. An improved process of this type is described in detail in co-pending U.S. Patent application Ser. No. 312,173, William A. Kolb et al, filed Dec. 4, 1972 and assigned to United States Steel Corporation.
A converter employed in carrying out this improved method comprises a tiltable vessel having a refractory lining and a bottom plug member provided with a plurality of nozzles, or tuyeres, extending through the bottom plug member. Each tuyere consists of a center jet through which oxygen flows during the refining portion of the process and an annulus jet concentrically surrounding the center jet through which a fuel gas flows to provide cooling for the tuyere to maintain the erosion of the tuyere substantially equivalent to that of the adjacent plug refractory.
Although oxygen is used in the center jet during the refining operation, various combinations of gases are required for purging or cooling the tuyeres and during other parts of the process, such as charging the converter, sampling the resulting melt, tapping the converter after the iron has been refined and during the transition periods when the converter is being rotated to a position in which the next operation can take place. With the converter on its side during the charging, sampling and tapping operations, the tuyeres may be protected from melting by the introduction of gases, such as compressed air at the center jets and low pressure nitrogen at the annulus jets. When the vessel is being raised to its upright position for the refining operation, the pressure at the jets must be increased to assure that the molten metal will not enter the tuyeres, thereby blocking the openings and allowing them to come into contact with the steel and highly corrosive slag. Nitrogen, at a relatively high pressure, may be substituted for the compressed air during this portion of the cycle.
After the converter is in its upright position and located under a hood which carries the gases away, the refining operation is carried out by substituting oxygen for the nitrogen at the center jet and a fuel for the nitrogen at the annulus jet. The pressure during refining must be high enough to prevent the nozzles from becoming blocked or damaged by contact with the melt. During the Q-BOP refining operation, various types of fluxes, such as lime, etc., are injected into the oxygen flow being fed to the center jet of the tuyeres. These fluxes are, of course, basic to the steelmaking process and are employed in required amounts to give the finished steel its desired characteristics of strength, durability, malleability, etc. In order that the finished steel will have the proper characteristics, it is necessary to accurately inject appropriate amounts of flux into the molten metal in the converter. To do this in the Q-BOP process requires a control system which will allow accurate control of the flux being fed into the oxygen flow for injection into the molten metal bath through the tuyeres of the steelmaking vessel.
Heretofore in conventional steelmaking processes such as the open hearth, electric furnaces, and BOP shops, the desired amount of fluxes are weighed out or batched into a hopper. When the desired amount of flux is in the hopper, a gate is opened and all the flux is dumped into the vessel at one time. In the Q-BOP process no space is available for storage bins and hoppers over the vessel as in conventional BOP shops. The various fluxes are therefore injected into an oxygen stream and thus carried into the vessel through the tuyeres. Various methods can be used to inject the flux into the Q-BOP converter. A co-pending patent application to William A. Kolb et al, Ser. No. 354,610, filed Apr. 25, 1973, assigned to United States Steel Corporation, and titled "Method and Apparatus for Controlling the Injection of Flux into a Steelmaking Vessel as a Function of Pressure Differential," now U.S. Pat. No. 3,871,633 describes an automatic control system for the injection of fluxes into the Q-BOP vessel by maintaining a fixed outlet orifice on the flux tank and controlling the flow of flux via means of adjusting tank pressures. The present invention pertains to an automatic control system for the injection of the fluxes into the Q-BOP vessel by maintaining a fixed tank pressure and varying the size of the outlet orifice. In either system the manual control of the rate of flux feed becomes difficult to maintain at the desired level. The operator may not react in time or may overreact in making corrections for the continuously changing rate. Also, the manual system requires the operator's constant attention in order to maintain a given flow rate, to change from one type of flux or flow rate to another, or to take preventative measures in case the flow becomes clogged.
This invention is therefore intended to provide an automatic control system for a flux injection apparatus having a variable orifice for adjusting the flow rate, which system starts the system by automatically opening the control valves in the system in proper sequence, automatically regulates the flow of material (to the rate set by the operator), automatically stops the flow of flux when the required amount (preset by the operator) has been injected into the vessel, automatically starts another tank preset to a different rate or having a different type of flux when the first tank has delivered its set weight, automatically monitors pressure to determine clogging and automatically stops flux flow when this occurs, and automatically opens a by-pass valve in the event of a failure in the system.